Which device transmits incoming data to the receiving device by using MAC addresses?

Data Link Layer

The data link layer (Layer 2) of the OSI model actually consists of two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The MAC sublayer controls device interaction. The LLC sublayer deals with addressing and multiplexing. Physical addressing for network connections exists at the data link layer. The data link layer combines data bits into entities called frames. Network topologies like Ethernet exist at the data link layer. Network switches are the most common network devices that exist at the data link layer.

Data Link Layer

The data link layer handles such tasks as gathering up sets of bits for transmission as packets and making sure the packets get from one end to the other. In addition, recognizing that physical layer transmission sometimes introduces errors, the data link layer handles error detection (and sometimes correction).

When the OSI model was first introduced, data communication was point to point. With time, data communication has become much more multipoint to multipoint, so the data link layer was divided to recognize multipoint needs. With the split of the data link layer, the traditional functions of the layer went to the upper portion, called the logical link layer. A new sublayer, media access control (MAC), was defined. The MAC layer routes packets from a sender to a receiver along a common path. It makes sure the message arrives at the intended recipient. The MAC layer adds a physical address, defining the intended recipient machine, and controls shared access to a resource. For example, control of TDMA operation (see Section 4.3.1) is a MAC layer function.

Data Link Layer

The data link layer is an interface between the network and physical layer. It is further subdivided into two modules: Medium Access Control (MAC) and Logical Link Control (LLC). The MAC module plays a critical role in conserving network life by efficiently allocating medium access to the contending nodes. The LLC is on top of the MAC layer and is responsible for cyclic redundancy check (CRC), sequencing information, and addition of appropriate source and destination information. The data link layer is also responsible for the multiplexing of data streams and data frame detection. So, with the preceding in mind: first create a network infrastructure, which includes establishing communication links between possibly thousands of nodes, and provides the network self-organizing capabilities. Second, the data link layer can fairly and efficiently share communication resources between all the nodes.

Data Link Layer

The data link layer is an interface between the network and physical layer. It is further subdivided into two protocol sublayers: medium access control (MAC) and logical link control (LLC). The MAC module has a critical role in conserving network life by efficiently and fairly allocating medium access to the contending nodes. The LLC is on top of the MAC layer and is responsible for cyclic redundancy check, sequencing information, and adding appropriate source and destination information. In addition, the data link layer is responsible for multiplexing of data streams and data frame detection.

2. The Data link layer

The Data link layer describes the logical organization of data bits transmitted on a particular medium. This layer defines the framing, addressing, and check-summing of Ethernet packets. The main task of the Data link layer is to transform a raw transmission facility into a line that appears free of transmission errors in the Network layer. It accomplishes this task by having the sender break the input data up into data frames (typically, a few hundred bytes), transmit the frames sequentially, and process the acknowledgment frames sent back by the receiver. Since the Physical layer merely accepts and transmits a stream of bits without any regard to meaning of structure, it is up to the Data link layer to create and recognize frame boundaries. This can be accomplished by attaching special bit patterns to the beginning and end of the frame. If there is a chance that these bit patterns might occur in the data, special care must be taken to avoid confusion. The Data link layer should provide error control between adjacent nodes.

Another issue that arises in the Data link layer (and most of the higher layers as well) is how to keep a fast transmitter from “drowning” a slow receiver in data. Some traffic regulation mechanism must be employed in order to let the transmitter know how much buffer space the receiver has at the moment. Frequently, flow regulation and error handling are integrated for convenience.

If the line can be used to transmit data in both directions, this introduces a new complication for the Data link layer software. The problem is that the acknowledgment frames for A to B traffic compete for use of the line with data frames for the B to A traffic. A clever solution in the form of piggybacking has been devised.

Data Link Layer (2)

The data link layer takes the packet from the network layer and breaks it into frames. The header in this layer provides the source and destination MAC addresses. It is the data link layer that will convert the data into binary digits such as 1 and 0 and then prepare them for the physical layer. This layer has to be aware of what type of network interface card (NIC) is being used in order to prepare the packet in a certain way. A frame prepared for Ethernet format would not be understood by a network set up with Token Ring. Thus, this layer takes the network interface into consideration before con-verting the packet. Cyclic redundancy Checking (CRC) is another feature found in the data link layer that provides the ability to detect if a received frame was damaged. This checking feature is normally done by the LAN switch or WAN frame relay switch.

Layer 2 devices that operate at this level are switches and bridges. They work by guiding the traffic to a destination based on the MAC address. The MAC address is a unique series of numbers and letter used to identify a certain network card. They are sometimes referred to as the physical address because this address is hard coded into the network card. A switch can direct traffic to the correct computer only if it's aware of what port the computer's network card is attached. This is done by the computer presenting the MAC address from its network card to the switch when it first comes online.

There are a variety of protocols that work at this layer. Some are used by hosts and others by network devices such as switches. STP (Spanning tree protocol) and RSTP (Rapid spanning tree protocol) are examples of protocols used by switches in this layer. They provide the ability to make sure there is only one layer 2 path to get to a destination. PPP (Point-to-point protocol) and L2TP (Layer 2 Tunneling protocol) are used by hosts. PPP provides the ability for a host to make a connection with a remote side using a modem. L2TP allows a host to connect to a remote side using a secure connection.

Layer 2: Data Link

The data link layer is responsible for maintaining the data link between two hosts or nodes. Its characteristics and functions are as follows:

Defines and manages the ordering of bits to and from data segments called packets

Management of frames, which contains data arranged in an organized manner, which provides for an orderly and consistent method of sending data bits across the medium

Responsible for flow control, which is the process of managing the timing of sending and receiving data so that it doesn't exceed the capacity of the physical connection

Responsible for error notification, including receiving and managing error messaging related to physical delivery of packets

Network devices that operate at this layer include Layer 2 switches (switching hubs) and bridges.

The data link layer is divided into two sublayers:

Logical Link Control (LLC) sublayer provides the logic for the data link. Thus, it controls the synchronization, flow control, and error checking functions of the data link layer.

Media Access Control (MAC) sublayer provides control for accessing the transmission medium. It is responsible for moving data packets from one network interface card (NIC) to another, across a shared transmission medium. Physical addressing is handled at the MAC sublayer. MAC is also handled at this layer. This refers to the method used to allocate network access to computers and prevent them from transmitting at the same time, causing data collisions. Common MAC methods include Carrier Sense Multiple Access/Collision Detection (CSMA/CD), used by Ethernet networks, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), used by AppleTalk networks, and token passing, used by Token Ring and Fiber Distributed Data Interface (FDDI) networks.

EXAM WARNING

A MAC address consists of six hexadecimal numbers. The highest possible hexadecimal number is FF:FF:FF:FF:FF:FF, which is a broadcast address. The first three bytes contain a manufacturer code and the last three bytes contain a unique station ID. You can view the MAC address on most systems with the following commands.

Windows ME, 9x: winipcfg (navigate the graphical user interface (GUI) to find the MAC address)

Windows NT, XP, Vista, 2000, 2003, 2008: ipconfig/all

Linux: ifconfig -a

3.1.5 Layer 3: Network Layer

The data link layer implements a packet link between computers attached to a common link. As we explained in our discussion of store-and-forward packet switching (see section 2.6.2), when computers are connected by a collection of point-to-point links, they must figure out where to send the packets that they receive: whether to send them out over another link and, if so, which one. (See Figure 3.6.) This function–finding the path the packets must follow–is called routing. Routing is one of the main functions of the network layer. The network layer appends unique network addresses of the source and destination computers. An important addressing scheme in packet-switched networks is that used by the Internet. Circuit-switched networks, like the telephone network, use different addressing schemes.

Thus, the network layer uses the transmission over point-to-point links provided by the data link layer to transmit packets between any two computers attached in a network.

Figure 3.7 shows a router attached to several links. When the router receives a packet, it must decide on the basis of the network addresses along which link it should retransmit the packet. This routing function is implemented by the network layer.

Observe that the link between, say, C and D in Figure 3.7 may carry packets between A and E and between B and E. These packets are statistically multiplexed by the router C (see section 2.6.1). The network addresses of the packets permit demultiplexing. Network layer standards specify packet formats, addressing schemes, and routing protocols.

21.5.3 IEEE 802.11 Data Link Layer

The data link layer within 802.11 consists of two sublayers: logical link control (LLC) and media access control (MAC). 802.11 uses the same 802.2 LLC and 48-bit addressing as the other 802 LAN, allowing for simple bridging from wireless to IEEE wired networks, but the MAC is unique to WLAN. The sublayer above MAC is the LLC, where the framing takes place. The LLC inserts certain fields in the frame such as the source address and destination address at the head end of the frame and error handling bits at the end of the frame.

The 802.11 MAC is similar in concept to 802.3, in that it is designed to support multiple users on a shared medium by having the sender sense the medium before accessing it. For the 802.3 Ethernet LAN, the carrier sense multiple access with collision detection (CSMA/CD) protocol regulates how Ethernet stations establish access to the network and how they detect and handle collisions that occur when two or more devices try to simultaneously communicate over the LAN. In an 802.11 WLAN, collision detection is not possible due to the near/far problem (see Chapter 11). To detect a collision, a station must be able to transmit and listen at the same time, but in radio systems the transmission drowns out the ability of a station to hear a collision.

4.2.1 Data Link Layer

Some data link layer protocols offer capabilities to save energy. This is especially important for mobile devices (like laptops or sensor nodes) that are equipped with and rely on (rechargeable) battery packs. One example for such a data link protocol that could easily be enhanced for energy-aware mechanisms are the IEEE 802.11 standards. 802.11 is a protocol widely used in the field of wireless data transmission. A lot of energy has to be used for the transmission of data, but also for receiving and even for just sensing the wireless channel. In infrastructure mode, a wireless node can notify the access point of a 802.11 network that it will go to sleep mode. This indication can be done by setting a special flag inside the 802.11 header and thereby telling the access point that the node will not listen for any further frames until the next beacon frame is sent. A beacon frame is usually sent every 100 ms, and the wakeup time of a node is approximately 250 μs. When the access point receives a frame that indicates sleep mode of a specific node, it will buffer all packages that it should send to the node and transmit it as soon as the node becomes available again. Of course, if there are no frames that were buffered in the mean time, the node could go to sleep again and save some energy [8].